Calorimetric loads have always been useful elements for testing RF equipment. This class of load converts RF wave energy into thermal energy that can be removed by a circulating coolant such as water. The power delivered to the load is measured by monitoring the temperature change of the circulating fluid, and is determined by the formula EQU Q=f.DELTA.TCp,
Where Q=heat removed (Watts),
Cp=specific heat of the circulating fluid (cal/g.degree.C.),
.DELTA.T is the temperature difference between input and output ports of the load in .degree.C.,
And f=flow rate of the fluid (1/min).
As an example, a 1.degree. C. rise in a fluid having a flow rate of 1 liter/minute indicates a power dissipation of 66 watts. There are several mechanisms wherein radio frequency energy is converted into heat. The principal class involves bulk resistive materials, wherein a material having E/H wave resistive properties is applied to a surface. Incoming waves are attenuated as they travel through this material until they are encounter the reflective surface. A typical choice for such a resistive material is carbon powder, which has a bulk resistance roughly of p=3000 .OMEGA.-cm.
A second mechanism for the dissipation of RF energy is the creation of magnetic eddy currents. This requires the use of a bulk ferrite, or other material magnetically coupled to short wave RF, which interacts with and attenuates the RF wave energy through the creation of eddy currents at the discontinuous interface. These eddy currents produce an electric field, which is dissipated in the resistivity of the absorbing material.
A third mechanism is the interaction of the incoming waves with a material which has a high dielectric loss for the applied wave frequency. At radio frequencies of above 10 Ghz, the absorption of water is sufficiently high to enable the use of water both as a coolant and load fluid. Another problem relative to high power density loads is the creation of very high heat loads and the dissipation and temperature limits encountered in removing the resultant heat load. An example of adding structures to the load to assist in heat removal is U.S. Pat. No. 3,904,993 High Power Microwave Load by James which discloses a load comprising a chamber with a lossy material. This chamber is further divided into sections which reduce the maximum internal voltage levels, as well as provide a heat conduction path for removing heat from the lossy material and directing it to the waveguide exterior.
An attenuative load utilizing the dielectric loss of water is discussed in U.S. Pat. No. 4,593,259 by Fox et al. his patent discloses a load comprising a conical reflector which is surrounded by a material having a high loss tangent dielectric constant fluid such as water. The conical reflector serves to direct input waves onto a multi-reflection path through the attenuative water media, so that a minimum of reflected wave energy is directed back to the wave source. One of the disadvantages of the use of water as the direct attenuative material is the initial discontinuity in impedance between the waveguide and the water interface. This discontinuity will result in the reflection of some fraction of the incoming wave power according to the relationship ##EQU1## Where Pr=power reflected back to waveguide
Pi=incident power from waveguide PA1 Zr=reflection interface impedance PA1 Zo=waveguide impedance PA1 750 W/1000 W=1.2 dB loss on first reflection, PA1 500 W/750 W=1.7 dB loss on second reflection, PA1 250 W/500 W=3 dB loss on third reflection, PA1 2.5 W/250 W=20 dB loss on fourth reflection.
The effect of this interface is that some fraction of the wave energy will be returned to the source, and cause standing waves to appear in the waveguide, thereby producing excessive electric fields within the waveguide, and ultimately limiting the maximum input power. U.S. Pat. No. 5,015,943 by Mako et al describes a means for measuring microwave radio frequency waves by measuring the thermal expansion of the heated material, either by using directly optical fiber in conjunction with an interferometer, or through the use of a linear motion transducer such as a piezoelectric crystal. This method limits the input power level of the RF load to the deformation temperature of the fiber optic sensor, although it can be accurate for lower temperature measurements associated with low incident power.